FIG. 38 and FIG. 39 each show a sectional view of the essential part of generally used die casting machines. In the case of the horizontal die casting machine of FIG. 38, molten metal 31 taken by a ladle 37 from a holding furnace is poured into a casting sleeve 30 from an inlet. The poured molten metal 31 is injected at a low speed by a plunger chip 33 in an early stage and charged into a cavity 36 formed by closing a die 34 and a die 35 through a high-speed injection in a later stage. In case of the vertical die casting machine of FIG. 39, molten metal 31 is poured into a cup 38, dies 34 and 35 are closed and the molten metal 31 is charged into a cavity 36 by the same way as in the case of the horizontal die casting machine. In FIG. 39, the components same with those of FIG. 38 are given the same reference numerals as used in FIG. 38, and their description is omitted.
The casting sleeve is generally kept at a low temperature to keep its machine accuracy and to prevent oxidation. This causes partial solidification of the molten metal when pouring. If solidified pieces are supplied into the cavity together with the molten metal, casting defects are caused and mechanical properties of a product may be deteriorated.
The casting sleeves, which are in a single-layered integral form made of metal, e.g., stainless steel, as shown in FIG. 38 and FIG. 39, have such a high heat conductivity that the molten metal is quickly cooled and viscosity of the molten metal is increased to lower fluidity making forced charging by a plunger difficult.
In conventional die casting, well-known methods include rheocasting and compocasting which vigorously stir a semi solid metal or composite material to break dendrite structures so as to continuously produce a slurry-state metal having lowered viscosity and feed the slurry-state metal into the die casting machine, and thixocasting and other techniques which once solidify a slurry-state metal and reheat it into a semi solid state to feed into the die casting machine.
In the above cases, the casting sleeve is also kept at a low temperature as described above, the molten metal is cooled and its viscosity is increased, lowering fluidity. Therefore, these casting processes cannot be used to produce a thin and long member because short runs and cold shuts occur in such casting processes. In these prior art processes, the dendrite structure may remain on the surface layer of the molten metal as disclosed in Japanese Patent Publication No. 2-51703, because these processes are designed to prevent the surface layer from entering into a product. In Japanese Patent Application Laid-open Prints No. 3-221253 and No. 3-13260, the surface of a molten metal is prevented from entering into a product, because the material surface is oxidized when preheated prior to being melted. Although the dies and the die casting methods are specially devised, good results may not be always obtained.
To remedy the above problems, Japanese Patent Publication No. 54-43976 discloses a die casting machine whose casting sleeve is made of a heat-resistant material such as a ceramic material.
The casting sleeve has its inner cylinder made of a heat-resistant material such as a ceramic or cermet, and its outer cylinder is shrinkage-fitted or internal-chilled with a reinforcing member made of iron, cast iron, cast steel or an ultra heat resistant alloy, such as a tungsten group alloy or a molybdenum group alloy, to apply a compressive stress to enhance the mechanical strength of the casting sleeve. The die casting machine is provided with a cooling means forcibly cooling the outer periphery of the reinforcing member partly or entirely with water or air, thereby permanently retaining the compressive stress against the casting sleeve.
Referring to FIG. 40, the above prior art is described in detail. A cavity 44 is formed by closing a die (movable die) 41 and a die (stationary die) 42 fixed to a main body (die plate) 43 of the die casting machine. A casting sleeve 45 is fixed to the die (stationary die) 42 in communication with the cavity 44. This casting sleeve 45 is made of ceramics or cermet so as to excel in heat resistance, corrosion resistance and abrasion resistance, hardly become wet with molten metal, and have a low thermal conductivity. The casting sleeve 45 has an inlet 46 formed and a plunger 47 is slidably disposed within the casting sleeve 45.
The molten metal poured through the inlet 46 is once stored in the casting sleeve 45, then the plunger 47 advances swiftly to charge the molten metal into the cavity 44 under pressure, and the charged molten metal is pressurized by the plunger 47 and solidified. After the molten metal solidification, the die 41 and the plunger 47 move back to provide a die-cast product.
Even in the above die casting machine, however, there are disadvantages, because the molten metal is still cooled by the inner cylinder of the casting sleeve to produce solidified pieces which may cause casting defects degrading the mechanical properties of a product if these solidified pieces are supplied into the cavity together with the molten metal. When the above die casting machine is applied to the rheocasting method, compocasting method or thixocasting method, the molten metal is cooled by the inner cylinder, so that short runs and cold shuts still occur in casting a thin and long member making the application of the machine difficult.
Japanese Patent Publication No. 6-83888 discloses a die casting machine which applies a high-frequency current to oscillating coils so as to cause electromagnetic induction, thereby retaining the molten metal in a non-contact state with respect to the injection sleeve wall and the plunger chip end face. Since the molten metal is not contacted with the injection sleeve wall and the plunger chip end face, the molten metal is heat-insulated resulting in prevention of an initial solidified layer and defective cold shuts.
In FIG. 41 showing the above die casting machine, the molten metal is poured into a runner 52 with a plunger chip 50 positioned at the lower part in an injection sleeve 51, a high-frequency current of 1000 Hz is supplied from a power unit to oscillating coils 54 and 55 which are respectively disposed at a wall 53 of the sleeve 51 and within the plunger chip 50, a repulsion force is generated between the molten metal and the wall 53 or the plunger chip 50 by the action of electromagnetic induction because of the conductivity of the molten metal, and the molten metal is held in a floated state within the sleeve 51 due to this repulsion force.
The above die casting machine has difficulty in precisely holding the molten metal in a non-contact state with respect the wall 53 or the plunger chip 50 by the action of a high-frequency current. To conduct secure holding of the molten metal in a non-contact state, the range for setting an a.c., e.g. the band for setting the frequency of an a.c., may be limited. This may cause the electromagnetic stirring for the molten metal to be insufficiently conducted.
In addition, the sleeve 51 is liable to be deformed by heating in the process of above electromagnetic induction by the molten metal, resulting in deterioration of the proper fitting of the plunger chip 50 into the sleeve 51. If the sleeve 51 is cooled to avoid the deforming caused by the induction heating, the molten metal in the sleeve 51 is hardly heated.